Fix running rendering tests if GZ_ENABLE_RELOCATABLE_INSTALL is ON

[traversaro]

🦟 Bug fix

If GZ_ENABLE_RELOCATABLE_INSTALL is ON, for all tests the environment variable GZ_RENDERING_INSTALL_PREFIX=${CMAKE_INSTALL_PREFIX} need to be defined, to make sure that even if the shared library in the build directory is used instead of the install prefix, the correctly installed engine plugins are loaded. As part of #804 I modified some tests to set GZ_RENDERING_INSTALL_PREFIX=${CMAKE_INSTALL_PREFIX}, but I forgot to do this modification in gz_configure_rendering_test, that is actually how most tests in gz-rendering are added.

Summary

Checklist

• Signed all commits for DCO
• Added tests
• Updated documentation (as needed)
• Updated migration guide (as needed)
• Consider updating Python bindings (if the library has them)
• codecheck passed (See contributing)
• All tests passed (See test coverage)
• While waiting for a review on your PR, please help review another open pull request to support the maintainers

Note to maintainers: Remember to use Squash-Merge and edit the commit message to match the pull request summary while retaining Signed-off-by messages.